JPH0727287B2 - Multicolor image forming method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrophotographic multicolor image forming method, and more particularly to a so-called one-shot multicolor image forming method for forming a multicolor image by one transfer. Regarding improvement.

(Prior Art) In an electrophotographic image forming method for forming a multicolor image of two or more colors, the surface of an image carrier (hereinafter referred to as a photosensitive drum) is initially set to a predetermined potential. After charging, a negative optical image exposure corresponding to the first color and development with the first color toner are performed, and then a negative optical image exposure corresponding to the second color and development with the second color toner are performed. There is a method of sequentially repeating the same operation for the required number of colors. This method will be described with reference to the multicolor image forming apparatus shown in FIG. 6 and the principle explanatory view of FIG. 7. First, the charging of the first image forming portion P1 arranged close to the photosensitive drum 1 shown in FIG. As shown in FIG. 7 (1), the surface of the photosensitive drum 1 is charged by the container 2 to a predetermined surface potential V _D , and a negative image exposure is performed by the first image exposure means 3, as shown in FIG. 7 (2). Thus, the negative latent image _VL of the first color is formed on the photosensitive drum 1.
Next, the first developing device 4 deposits the first color toner Ta on the area of the negative latent image _VL of the first color as shown in FIG. 7C.

Next, in the second image forming portion P ₂ shown in FIG.
Image exposing means to form a second negative latent image _VL as shown in FIG. 7 (4), and a second developing device to apply the second color toner Tb to the second latent image _VL as shown in FIG. 7 (5). To the area of the negative latent image V _L. Hereinafter, the same operation is performed by the third image forming unit P ₃ . In FIG. 6, 5 is a transfer charger, 6 is a transfer paper, and 7 is a cleaning means.

Thus, such a multicolor image forming method has the advantage that different color toners of the same polarity can be used and the transferability to the transfer paper is good.

In addition to the above methods, the following methods are also available as the multicolor image forming method. That is, in this method, after the surface of the photosensitive drum is charged to a predetermined potential, a negative optical image exposure corresponding to the first color and a reversal development with the first color toner are performed, and then the second color is developed. In this method, a positive light image exposure corresponding to a color and a regular development with a second color toner are performed. This method will be described with reference to the two-color image forming apparatus shown in FIG. 8 and the principle explanatory view of FIG. 9. First, the charger 2 shown in FIG. ), The surface of the photosensitive drum 1 is charged to a predetermined surface potential V _D , negative image exposure is performed by the first image exposing means 3, and the first image exposing means 3 forms a negative image on the photosensitive drum 1 as shown in FIG. 9 (2). One latent image V ₁ is formed. Next, as shown in FIG. 9 (3), the first developing device 4 deposits the toner Ta of the first color on the area of the first color latent image V ₁ by reversal development. Incidentally, FIG. 9 (3)
Inside, V _DC1 indicates the developing bias voltage at the time of reversal development.

Next, a positive image exposure is performed by the second image exposure means 11, and the second image is exposed on the photosensitive drum 1 as shown in FIG. 9 (4).
A second developing device for forming a color latent image V ₂ as shown in FIG.
The second color toner Tb is subjected to the second
Adhere to the area of the color latent image V ₂ . In addition, in FIG. 9 (4), V
_DC2 indicates the developing bias voltage at the time of regular developing.

Thereafter, the pre-transfer charger 17 shown in FIG. 8 charges the toners Ta and Tb to the same polarity, and the transfer charger 5 transfers the toners Ta and Tb onto the transfer paper 6. The eighth
In the figure, 7 is a cleaning means and 16 is a fixing means.

Thus, such a two-image forming method has a large latent image contrast, and thus is advantageous for development and has an advantage that there is little color mixture on the image.

(Problems to be Solved by the Invention) However, in the former method, latent images of the same polarity are formed, so that, for example, when the image is developed by the second developing device,
There is a problem that the latent image of is also developed. this is,
Even after the completion of the first development, the first latent image is as shown in FIG.
As shown in FIG. 7, it is caused by having a potential difference ΔV _D with respect to the base potential V _{D. As} shown in FIG. 7 (5), the first latent image developed with toner Ta is further developed with toner Tb. As a result, so-called color mixture on the image occurs. To prevent this, a method of setting the developing bias voltage to the second developing device to a value that does not develop the first latent image can be considered, but according to this method, the developing contrast (developing bias voltage and exposure portion potential V _L Difference) between the first image forming step and the second image forming step, resulting in a difference in image density, a line width of a line image, and the like, resulting in deterioration of image quality. The potential difference ΔV _D shown in FIG. 7 (4) is slightly improved by applying AC charging with DC superposition prior to the second image forming step.
It has not reached satisfactory results.

In the latter method, the non-image portion potential of the second color
Since V _D becomes equal to the image portion potential V _D of the first color and the developing bias voltage V _DC2 of the second color is set to a value closer to the non-image portion potential V ₁ of the second color, the toner Ta of the first color is There is a problem that the adhesive force to the surface of the photosensitive drum 1 becomes weak, the toner image of the first color is disturbed during the development of the second color, and the toner Ta of the first color is peeled off and becomes thin.

Therefore, it is conceivable to use the non-contact development for the development of the second color, but if this non-contact development is carried out, the developing device becomes complicated and high-precision parts are required, resulting in an increase in cost. Further, it is extremely difficult to maintain stable and good development characteristics by this non-contact development.

(Object) The present invention has been made in view of the above conventional problems, and an object of the present invention is to provide a multicolor image forming method capable of obtaining a high-quality multicolor image.

(Summary of the Invention) In order to achieve the above object, the present invention provides a multicolor image forming method in which a multicolor image is formed by repeating an image forming process including image exposure and development and performing a single transfer process. The time from the image exposure to the development in the forming process was changed so that the surface potential of the image bearing member at each development was the optimum value for the development.

Thus, according to the present invention, it is possible to prevent the formation of a mixed color image by the different color toners, prevent the deterioration of the image quality due to the development of the image formed on the image bearing member in the next image forming step, and as a result, achieve the high quality It is possible to obtain the effect that a multicolor image of is obtained.

(Example) Hereinafter, an example of the present invention, particularly an example of a method of obtaining a multicolor image by repeating negative image exposure and development, will be described with reference to the accompanying drawings.

FIG. 1 is a block diagram of a two-color image forming apparatus for carrying out the method of the invention, and FIG. 2 is a diagram for explaining the operating principle of the apparatus.

First, a schematic structure of a two-color image forming apparatus will be described with reference to FIG. 1. In FIG. 1, reference numeral 1 denotes a photosensitive drum that is rotationally driven in a direction indicated by an arrow at a constant speed. Around 1, there is provided a charging device 2, a first image exposing means 8, a developing device 9 for developing the first color, a second image exposing means 11, a developing for developing the second color in the vicinity thereof. Device 12, transfer charger 5, separation charger 14, cleaning device 7, and static elimination lamp 15 for removing residual charges on the surface of the photosensitive drum 1.
Are sequentially arranged. Further, the developing devices 9 and 12 have a developing bias voltage V applied to the developing rollers 9a and 12a.
Voltage sources 10 and 13 for applying _DC1 and V _DC2 are attached, respectively.

By the way, the first image exposing means 8, the developing device 9, and the second
Distance on the photosensitive drum 1 between the image exposure means 11 and the developing unit 12 is set to l _1, l ₂ As shown, these two distances l _1,
l _2> l ₁ the relationship is maintained between the l _2. The first
In the figure, 6 is a transfer paper and 16 is a fixing device.

Next, the method of the present invention will be described with reference to FIG.

First, as shown in FIG. 2 (1), the surface of the photosensitive drum 1 is charged to a predetermined potential V _D by the charger 2. Here, for the sake of convenience, the photosensitive drum 1 is considered as a P-type semiconductor, and the potential V _D is positively charged.

Next, a negative image exposure is performed on the uniformly charged photosensitive drum 1 by the first image exposure means 8 to form a first latent image V _L1 as shown in FIG. 2 (2). afterwards,
As shown in FIG. 2 (3), the first developing device 9 located at the position l _{1 in} the surface distance of the photosensitive drum 1 from the position of the image exposing means 8 changes the toner Ta of the first color having a positive charge. Then, reversal development of the latent image V _L1 is performed. At this time, the developing bias voltage V _DC1 is applied to the developing roller 9a of the developing device 9 by the developing bias voltage source 10.
_DC1 is set to a value that is slightly lower than the potential V _D, and so-called fog does not occur in the background portion (non-image portion).

Then, since the toner Ta of the first color adheres to the first latent image V _L1 , the potential of the photosensitive drum 1 after development rises to V _L1 ′ as shown in FIG. 2 (3).

Then, as shown in FIG. 2 (4), the second image exposure means 11
To provide a negative image exposure on the photosensitive drum 1.

By the way, the photosensitive drum 1 has a light response characteristic as shown in FIG. 3, and when light is applied to the photosensitive drum 1 charged to a predetermined potential, the surface potential thereof decreases in a hyperbola from that moment. In FIG. 3, the horizontal axis represents time [sec] and the vertical axis represents surface potential [V]. The figure shows the photoresponse characteristics of an OPC photosensitive drum using a phthalocyanine pigment in the charge carrier generation layer.

Therefore, by changing the distance on the surface of the photosensitive drum from the image exposure means to the developing device, the time from image exposure to development can be changed. Therefore, from the characteristics shown in FIG. The surface potential of can be set to any value. Therefore, in the present embodiment, since l ₂ > l ₁ is set as described above, the potential of the second negative exposure portion is set.
As shown in FIG. 2 (5), V _L2 has a value lower than the potential V _{L1 of} the first negative exposure section.

After that, as shown in FIG. 2 (5), the developing bias voltage source 13
Developing bias voltage V _DC2 at developing roller 12a of developing unit 12
The second developing device 12 located at a position separated from the second negative exposing means 11 on the surface of the photosensitive drum 1 by a distance l ₂ while being applied to the second developing device 12
The second color toner Tb having a positive charge by the second
Reverse development of the latent image V _L2 of. The developing bias voltage V _DC2 applied at this time is set to a value shifted to 0 V from the developing bias voltage V _DC1 applied to the first developing device 9. That is, the potential V _L1 ′ in the first image forming step
The developing bias voltage V _DC2 may be set to a value slightly lower than the potential V _L1 ′ so that the second color toner Tb does not adhere to the partial area.

Here, it is desirable that the following relationships be established between the potentials V _D1 , V _DC1 , V _L1 , V _L1 ′, V _DC2 and V _L2 .

V _D −V _DC1 ≧ V _L1 ′ −V _DC2 V _DC1 −V _L1 ≦ V _DC2 −V _L2 The first and second color toner images are formed on the photosensitive drum 1 by the series of steps described above. This toner image is transferred onto the transfer paper 6 by the transfer charger 5. Then, the transfer paper 6 is separated from the surface of the photosensitive drum 1 by the separation charging device 14, and the toner image transferred onto the transfer paper 6 is fixed by the fixing device 16. On the other hand, the surface of the photosensitive drum 1 after the transfer is cleaned by the cleaning device 7, and then the charge is removed by the charge removing lamp 15, thus completing one step of the two-color image formation.

As described above, according to this embodiment, the distance between the first image exposure unit 8 and the developing unit 9 and the distance between the second image exposure unit 11 and the developing unit 12 on the photosensitive drum 1 is utilized by utilizing the light response characteristic of the photosensitive drum 1. Since l ₁ and l ₂ are changed and the time from image exposure to development in the image forming process is changed so that the surface potential of the photosensitive drum 1 during development can be set to an optimum value for the development, It becomes possible to obtain a high-quality multicolor image by preventing the formation of a mixed color image with different color toners.

Next, specific examples of the present invention will be described.

<Example 1> As for the apparatus, the process speed (peripheral speed of the photosensitive drum) is
100 mm / sec, the distances l ₁ = 15 mm and l ₂ = 30 mm shown in FIG. ₁ were used, and as the photosensitive drum, the organic photosensitive drum having the photoresponsive characteristics shown in FIG. 3 was used.

Then, after negatively charging about -700 V on the organic photoconductor drum, the first negative exposure corresponding to the image signal for black image was performed by the semiconductor laser. The potential of the exposed portion at this time was about -170V.

Then, a developing bias voltage of about -550 V was applied with black toner having a negative charge to perform reversal development, and then positive exposure corresponding to a red image signal was performed by a second semiconductor laser. At this time, the potential of the exposed part is about −
It became 130V. After that, reversal development was carried out by setting the developing bias voltage to about -500 V with red toner having a negative charge. As a result, it was possible to obtain a high-quality two-color image with no color mixture, and for both black and red solid images and line images, which are sufficiently satisfactory in terms of density and sharpness.

In the above embodiment, the semiconductor laser light was used for the first and second image exposures, but other LED exposures, exposures using a liquid crystal shutter, reflected light from a manuscript, a film, or the like, or transmitted light may be used. You can Also, the first and second
When a semiconductor laser is used for image exposure, the light from one laser light source may be divided into two by an A / 0 deflector, or two laser light sources may be used.

Next, another embodiment of the present invention, particularly an embodiment in which the first image exposure is a negative image exposure and the second image exposure is a positive image exposure, is an apparatus configuration diagram of FIG. 4 and a principle explanatory diagram of FIG. It will be described based on.

The two-color image forming apparatus shown in FIG. 4 is the same as that shown in FIG. 1, and here the first image exposing means 8 and the developing device 9 and the second image exposing means 11 and the developing device 12 are provided. The distance between them on the photosensitive drum 1 is set to l ₃ and l ₄ as shown in the figure, and a relationship of l ₃ > l ₄ is maintained between these two distances l ₃ and l ₄ .

Next, the image forming method according to this embodiment will be described with reference to FIG.

First, as shown in FIG. 5 (1), the surface of the photosensitive drum 1 is charged to a predetermined potential V _D by the charger 2. Here, for the sake of convenience, the photosensitive drum 1 is considered as a P-type semiconductor, and the potential V _D is positively charged.

Next, a negative image exposure is performed on the uniformly charged photosensitive drum 1 by the first image exposure means 8 to form a _first latent image V ₁ as shown in FIG. 5 (2). After that, as shown in FIG. 5C, the latent image V ₁ is reversely developed by the first developing device 9 with the toner Ta of the first color having a positive charge. At this time, the developing bias voltage V _DC1 is applied to the developing roller 9a of the developing device 9 by the developing bias voltage source 10, but the developing bias voltage V _DC1 is slightly lower than the potential V _D , so that the background portion (non-image portion) It is set to a value at which so-called fogging does not occur.

Then, as shown in FIG. 5 (4), the second image exposure means 11
To provide a positive image exposure on the photosensitive drum 1 by the second
After forming the latent image V ₂ of the second developing device 12 while applying the developing bias voltage V _DC2 to the developing roller 12a of the developing device 12 by the developing bias voltage source 13 as shown in FIG. Thus, the second latent image V ₂ is normally developed with the second-color toner Tb having a negative charge. The developing bias voltage V _DC2 applied at this time is set to a value that does not cause fogging on the background portion.

Thus, in this embodiment, as described above, the relationship of l ₃ > l ₄ is maintained between the distances l ₃ and l ₄ , so that the non-image portion of the second color shown in FIG. Has a value higher than the potential V ₁ of the image portion of the first color, and the toner Ta having the positive charge of the first color is attached to the latent image V ₁ of the first color even during the development of the second color. The adhesion force is stronger than the adhesion force to the non-image portion of the second color around which the toner Ta is attached, so that the toner Ta is not disturbed or peeled off, and the first color Image quality deterioration due to the development of the second color of the image is effectively prevented, and a high-quality two-color image is obtained.

By the series of steps described above, the toner images of the first and second colors are formed on the photosensitive drum 1, and then the toner images are transferred to the transfer paper 6 by the transfer charger 5. Then, the transfer paper 6 is separated from the surface of the photosensitive drum 1 by the separation charging device 14, and the toner image transferred onto the transfer paper 6 is fixed by the fixing device 16. On the other hand, the surface of the photosensitive drum 1 after the transfer is cleaned by the cleaning device 7, and then the charge is removed by the charge removing lamp 15, thus completing one step of the two-color image formation.

Next, specific examples of the present invention will be described.

<Embodiment 2> As for the apparatus, the process speed (photosensitive drum peripheral speed) is
100 mm / sec, the distances l ₃ = 30 mm and l ₄ = 15 mm shown in FIG. 1 were used, and as the photosensitive drum, the organic photosensitive drum having the photoresponsive characteristics shown in FIG. 3 was used.

Then, after negatively charging about -700 V on the organic photoconductor drum, the first negative exposure corresponding to the image signal for black image was performed by the semiconductor laser. The potential of the exposed portion at this time was about -130V.

Next, a developing bias voltage of about -500 V was applied with black toner having a negative charge to perform reversal development, and then positive exposure corresponding to a red image signal was performed by a second semiconductor laser. At this time, the potential of the exposed part is about −
It became 170V. After that, regular development was carried out by setting the developing bias voltage to about -300 V with red toner having a positive charge. As a result, it was possible to obtain a high-quality two-color image in which the toner of the black image was not disturbed and the colors were not mixed, and the solid image of black and red and the line image were sufficiently satisfactory in terms of density and sharpness.

Although the above is limited to the two-color image forming method,
According to the method of the present invention, it is needless to say that a multicolor image of two or more colors can be similarly obtained.

(Effects of the Invention) As is apparent from the above description, according to the present invention, a multicolor image forming method for forming a multicolor image by repeating an image forming process including image exposure and development and performing one transfer process is performed. ,
By changing the time from image exposure to development in each image forming process so that the surface potential of the image carrier at each development becomes the optimum value for the development, it is possible to prevent the formation of mixed color images by different color toners, It is possible to prevent deterioration of image quality due to development in the next image forming step of the image formed on the carrier, and obtain an effect that a high-quality multicolor image can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of an image forming apparatus for carrying out the method of the present invention, FIG. 2 is an explanatory view of the principle of the method of the present invention, FIG. 3 is a view showing a photoresponse characteristic of a photosensitive drum, and FIG. The block diagram of the image forming apparatus for carrying out the method of the present invention according to the embodiment,
FIG. 5 is an explanatory view of the principle of the method of the present invention, FIG. 6 is a block diagram of an image forming apparatus for carrying out the conventional method, FIG. 7 is an explanatory view of the principle of the conventional method, and FIG. 8 is another conventional method. FIG. 9 is a configuration diagram of an image forming apparatus for carrying out the method, and FIG. 9 is an explanatory view of the principle of the conventional method. l ₁ , l ₂ , l ₃ , l ₄ ...... Distance on the surface of the image carrier from the image exposing means to the developing device, 1 ... Photosensitive drum, 2 ... Charging device,
5 ... Transfer electric device, 6 ... Transfer paper, 8 ... First image exposing device, 9 ... First developing device, 11 ... Second image exposing device, 12 ... Second developing device .

Claims (4)

[Claims] 1. A multicolor image forming method, wherein an image forming process including image exposure and development is repeated to form a multicolor image through one transfer process, from image exposure to development in each image forming process. Is changed so that the surface potential of the image bearing member at each development has an optimum value for the development. 2. A multicolor image forming method for forming a multicolor image through one transfer step by repeating an image forming step including negative image exposure and development, from image exposure to development in a certain image forming step. The multicolor image forming method according to claim 1, characterized in that the time required to reach the time is set longer than that in the preceding image forming step. 3. A multicolor image in which the image exposure in the first image forming step is a negative image exposure and the image exposure in the second image forming step is a positive image exposure, and a two-color image is formed through one transfer step. The multicolor according to claim 1, wherein in the forming method, the time from image exposure to development in the first image forming step is set longer than that in the second image forming step. Image forming method. 4. The time from image exposure to development is changed by changing the distance on the surface of the image carrier from the image exposure means to the developing device. The multicolor image forming method according to any one of items 1 to 3.